In the manufacture of semiconductor devices, it is standard practice to coat the leads of packaged devices with a thin layer of a corrosion resistant and aesthetically pleasing coating material. This coating material is commonly referred to as the lead finish. A commonly used lead finish is reflowed tin which is provided by first electrolytically depositing tin onto the leads, commonly called "plating", which results in a matte finish, and then heating the thus coated leads to the tin's melting point. The melting of the tin is referred to as "reflow" and it results in the conversion of the matte finish to a shiny finish.
The reflow process may be conducted in a variety of manners. The particular option of interest in the present invention is known as vapor phase reflow which has become a widely utilized method due to the relative uniformity of quality obtainable in providing a high quality lead finish. This uniformity results from either the batch or continuous mode operations although the continuous, or "in line," systems tend to provide the best uniformity of quality. The invention deals with the in line process.
In its simplest form, an in line process includes a conveyance mechanism for providing movement of a series of packaged chips through a reflow region or chamber. As the chips pass through the reflow chamber the previously plated leads are heated until the tin is melted, at least at its surface, and the matte finish converts to a shiny finish. The chips then are moved from the reflow chamber. The reflow chamber is typically constructed such that a boiling liquid can be held in the bottom of the chamber to establish a liquid/vapor equilibrium within the chamber and thus insure maintenance of a constant temperature.
In the ideal arrangement, the duration of the chips' passage through the reflow chamber is carefully timed such that reflow occurs immediately prior to the chips exit from the chamber. In practice, however, it has not been possible to consistently attain this theoretical optimum timing. As a result of the unavoidable variations in process parameters, albeit within fairly close tolerances, it has been necessary to insure that all chip leads obtain the desired reflow by designing the system such that the chip remains in the chamber longer than the theoretical minimum and long enough to statistically insure reflow on all leads. As a result, most of the leads not only reach the point where the surface become shiny, but also reach the point where the tin begins to flow from the leads. This phenomenon is referred to as tin "run off".
The conveyor mechanism which transports the chips through the reflow chamber is designed to permit free vapor flow around the leads in order that a good heat exchange may occur and the reflow thus may uniformly be achieved. FIG. 1 illustrates the typical arrangement wherein the reflow chamber 10 has a boiling liquid 12 in the lower portion and hot vapor 14 in equilibrium over the boiling liquid. A conveyor belt 50 is arranged to pass through portals 11a, 11b such that the belt 50 passes through the vapor contained in the upper portion of the reflow chamber. In an effort to provide minimum restriction of vapor circulation, the conveyor belt is constructed of an open grid mesh. The mesh may be made for instance of fiberglass strands coated with a non-stick coating such as Teflon. The open grid mesh may typically have four strands per inch to allow sufficient vapor flow. With such an open grid mesh, it is necessary to place the chips on the conveyor belt in a "bug up" orientation. That is, the leads point up and the package "top" is placed down on the belt. If the chips are placed on the belt "bug down", the leads penetrate the mesh, making contact with the mesh support and inlet and exit throats along the entire length of the machine. As the belt rounds the roller 16, the leads protruding through the belt can be bent or broken and frequently the presence of the protruding leads will cause jamming of the belt operation at the support and throat seams and joints. Also, visual blemishes on reflowed leads can occur due to contact of the lead surface with the belt mesh during reflow.
While the bug up arrangement has been required as a result of the need for an open mesh, it has been found that the bug up process produces a very thin reflowed coating due to the amount of tin run off which occurs in this arrangement. It has been discovered that a thicker reflowed tin layer can be provided by the process of the invention without interfering with the movement of the conveyor belt or creating optical blemishes. The thicker reflowed tin layer results in higher quality finished leads with improved solderability.
While it has been discovered that a bug down process provides better quality finished leads, it is to be noted that the mere adoption of a bug down arrangement is not sufficient. It is also necessary to provide for the free circulation of vapor throughout the chamber. Thus, a solid belt would be unacceptable in this regard. Use of a mesh of sufficiently smaller grid would restrict vapor flow less than a solid belt and might be able to support the chip without damaging the leads or interfering with belt rotation. However, the greater number of strands and the increase in the number of strand intersections would provide a proportionately greater surface area and an increase in the number of intersection locations where the reflow fluid would collect and be removed from the chamber. This removal of the reflow material from the chamber is known as "dragout". It is noted that the use of a solid belt would also be subject to increased drag out relative to the open mesh belt used in bug up systems. Because of the expense of maintaining the reflow chamber's fluid level, it is desirable to avoid excessive dragout of the reflow material.
Thus, it is an object of the present invention to provide an improved vapor phase tin reflow system wherein a chip can be conveyed bug down through the reflow chamber without interfering with the system operation.
It is also an object of the present invention to provide such a system which does not create any visually evident damage to the reflowed tin on the leads and does not structurally damage the leads.
It is another object of the present invention to provide this improved system without causing excessive dragout of the reflow material and without substantially reducing the circulation of the reflow vapor in the upper portion of the reflow chamber.